WMOD00500

• Title: Observations of Silver Iodide Plumes over the Grand Mesa of Colorado
• Date: 10/10/1988
• State: CO
• Weather Modification - Doc Type: Report

OcrOBER 1988

HOLROYD, McPARTLAND AND SUPER

1143

.;

by using complexes that are more effective at warmer
temperatures (e.g., DeMott et al. 1983). For the colder
storms, ground-released plumes of 30 g h -1 AgI-NH4I
should generate about 10 L -1 ice particles in the pres-
ence of supercooled liquid water. The behavior of both
the IN and ice particle plumes from these experiments
closely resembles that found in experiments over the
Bridger Range in Montana (Super 1974; Super and
Heimbach 1988). The results of the Grand Mesa and
Bridger Range plume tracing experiments, therefore,
might have more general application than at only the
two specific mountainous areas.
Based on the superior ice particle plume measure-
ments, an along-the-wind spreading rate within a factor
of three of about 2 m s -1 can be expected in cloudy
conditions with aircraft-released AgI, while a lesser rate
may apply to clear air. The vertical spreading rate was
not measured.
The ice particle plume diffusion measurements de-
scribed in this paper allow an estimation of the treatable
band width during cloud seeding by a single aircraft.
We will assume half of the full plume spreading rate
of 2.0 m S-1 for the lateral diffusion away from the
seeding line in each direction, an 80 m s -I aircraft
speed, a 10 m S-1 wind velocity, and an arbitrary 2000
s time period for the plume edges from successive lines
to touch. (By 2000 s most of the above experiments
were producing ice particle plumes near the crest of
the mesa.) In 2000 s the plume diffuses to a 4 km wide
zone centered on the flight track, which should there-
fore be the spacing between reciprocal flight tracks.
The 4 km plume drift under a 10m s -1 wind indicates
that a 400 s interval is needed between the tracks, dur-
ing which time the aircraft travels about 30 km, in-
cluding a turn. So a cross-wind band of about 30 km
could be treated by an aircraft passing back and forth
upwind of the mesa under normal conditions. The
variability in spreading rates means that sometimes the
cloudy volume will not be fully treated.
In both the ground- and aircraft-released AgI ex-
periments involving ice particle plumes, typical
achieved concentrations of ice particles were about 10
L -1 . This is similar to those achieved for the Bridger
Range (Super and Heimbach 1988). Future experi-
ments might try to push that number higher to see if
the cloud is limiting its production of ice particles, as
suggested by Fukuta (1985).
This series of experiments repeatedly demonstrated
that seeding materials can be properly delivered to in-
tended orographic cloud regions, both from ground
sites high up the windward slope and from aircraft up-
wind of the target. In many cases of both types, the
AgI produced an obvious plume of ice particles in sig-
nificant excess of background concentrations, and
usually smaller than background sizes because of
shorter growth times available. Super and Boe (1988)
elaborate on the microphysical effects of some of those
experiments. Super and Heimbach (1988) show that

similar results were obtained over a mountain range
about 800 km farther to the north.

Acknowledgments, This work was done as part of
the Bureau's Colorado River Augmentation Demon-
stration Program, under the direction of John Lease.
The climatological studies of data from the PROBE
stations and their relation to supercooled water were
performed by Bruce A. Boe. Boe and Jack T. Mc-
Partland were the flight scientists during these experi-
ments. John Thompson, Bill Hauze, and Marty Thorpe
ofNOIth American Weather Consultants released the
seeding materials and operated various equipment un-
der contract 5-CR-81-05090 with the Bureau of Rec-
lamation. Data systems were maintained by Michael
Collins. The aircraft from Aero Systems was skillfully
piloted by Dave Davalos. The Science Engineering As-
sociates aircraft data system was prepared by Lyle Lilie
and Michael Collins. Cindy Snook typed the manu-
script.

REFERENCES

Boe, B, A" and A. B, Super, 1986: Wintertime characteristics of
supercooled liquid water over the Grand Mesa of western Col-
orado, J, Wea, Mod., 18, 102-107,
DeMott, p, J" W, C, Finnegan and L. 0, Grant, 1983: An application
of chemical kinetic theory and methodology to characterize the
ice nucleating properties of aerosols used for weather modifi-
cation, J, Climate Appl. Meteor" 22, 1190-1203,
Egan, B., and F. A. Schiermeier, 1986: Dispersion in complex terrain:
A summary of the AMS workshop held in Keystone, Colorado,
17-20 May 1983, Bull. Amer. Meteor, Soc., 67,1240-1247,
Elliott, R. D., D, A, Griffith, J, A, Hueck and J, F, Hannaford, 1983:
Continuing design, review, and analysis work for the Sierra Co-
operative Pilot Project. Interim Rep, No, 6 (NA WC No, SL WM-
83-2) to Bureau of Reclamation, North American Weather
Consultants, Salt Lake City, 133 pp,
Finnegan, W, G" and R, L. Pitter, 1987: Rapid ice nucleation by
aCI:tone,silver iodide generator aerosols and implications to
winter orographic storm seeding strategies, Pre prints, 11th Con!
on Weather Modification, Edmonton, Amer, Meteor. Soc., 9-
10,
Fukuta, N" 1985: Application of slow acting contact-freezing nuclei
in ice-phase weather modification, Preprints, Fourth WMO Sci-
entific Con! on Weather Modification, Honolulu, WMO, 219-
224,
Garvey, D., 1975: Testing of cloud seeding materials at the Cloud
Simulation and Aerosol Laboratory, 1971-1973, J, Appl, Me-
teor., 14, 883-890,
Hill, G. E" 1980: Dispersion of airborne-released silver iodide in
winter orographic clouds, J. Appl. Meteor., 19, 978-985,
-, 1982: Evaluation of the Utah operational weather modification
program, Final Rep, Contract NOAA/NA-81-RAC-00023,
UWRL, USU, Logan, Utah, 291 pp,
Hobbs, p, V" L. F, Radke, J, R, Heming and D, G, Atkinson, 1975:
Airborne ice nucleus and cloud microstructure measurements
in naturally and artificially seeded situations over the San Juan
Mountains in Colorado, Research Rep, X, Dept. Atmos. Sci"
Univ, of Washington, Seattle, 89 pp,
-, and A, L. Rangno, 1985: Ice particle concentrations in clouds,
J. Atmos. Sci., 42, 2523-2549,
Holroyd, E. W" Ill, 1970: Prevailing winds on Whiteface Mountain
as indicated by flag trees, Forest Science, 16, 222-229,
-, 1987: Some techniques and uses of 2D-C habit classification
software for snow particles, J, Atmos, Ocean, Tech., 4,498-511,
Karacostas, T, S" 1981: Turbulent diffusion studies of winter storms